Add "difficulties" for competitive

[Modersmadre-2165]

@PhilDunphy-2404

There are many references on reddit and in the forums.

Here’s one from Prometheus back in Oct '19:

+3 -3 MMR under the hood discussion

I think you’re misunderstanding what the +/-3 value actually is. It’s a Standard Deviation from 0, which is about where 2500 SR is (this value can technically change over time, but it trends towards 2500). It in essence comes from the standard Gaussian function (Bell curve or normal curve) and how 99.7% of data points fall within 3 standard deviations of the mean. While SR is shown as a bimodal normal distribution (prior to decay being removed, it likely would be a proper normal curve now) MMR…